Osmium(IV) complexes for OLED materials

ABSTRACT

A novel compound having the formula Os(L) n  is disclosed, wherein Os is osmium(IV) metal, L is a ligand coordinating to the Os atom, and n is an integer from 1 to 6, wherein each L can be same or different, wherein at least one L is a multidentate ligand, and wherein the compound is neutral.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of U.S. application Ser. No. 16/014,091, filed Jun. 21, 2018, which is a divisional application of U.S. application Ser. No. 14/075,653, filed Nov. 8, 2013, now U.S. Pat. No. 10,069,090, which claims priority to U.S. Provisional Application No. 61/728,411, filed Nov. 20, 2012, the disclosure of which is incorporated herein by reference in its entirety.

PARTIES TO A JOINT RESEARCH AGREEMENT

The claimed invention was made by, on behalf of, and/or in connection with one or more of the following parties to a joint university corporation research agreement: Regents of the University of Michigan, Princeton University, University of Southern California, and the Universal Display Corporation. The agreement was in effect on and before the date the claimed invention was made, and the claimed invention was made as a result of activities undertaken within the scope of the agreement.

FIELD OF THE INVENTION

The present invention relates to compounds for use as emitters and devices, such as organic light emitting diodes, including the same. More particularly, the compounds disclosed herein are novel phosphorus containing bis(tridentate) osmium complexes.

BACKGROUND

Opto-electronic devices that make use of organic materials are becoming increasingly desirable for a number of reasons. Many of the materials used to make such devices are relatively inexpensive, so organic opto-electronic devices have the potential for cost advantages over inorganic devices. In addition, the inherent properties of organic materials, such as their flexibility, may make them well suited for particular applications such as fabrication on a flexible substrate. Examples of organic opto-electronic devices include organic light emitting devices (OLEDs), organic phototransistors, organic photovoltaic cells, and organic photodetectors. For OLEDs, the organic materials may have performance advantages over conventional materials. For example, the wavelength at which an organic emissive layer emits light may generally be readily tuned with appropriate dopants.

OLEDs make use of thin organic films that emit light when voltage is applied across the device. OLEDs are becoming an increasingly interesting technology for use in applications such as flat panel displays, illumination, and backlighting. Several OLED materials and configurations are described in U.S. Pat. Nos. 5,844,363, 6,303,238, and 5,707,745, which are incorporated herein by reference in their entirety.

One application for phosphorescent emissive molecules is a full color display. Industry standards for such a display call for pixels adapted to emit particular colors, referred to as “saturated” colors. In particular, these standards call for saturated red, green, and blue pixels. Color may be measured using CIE coordinates, which are well known to the art.

One example of a green emissive molecule is tris(2-phenylpyridine) iridium, denoted Ir(ppy)₃, which has the following structure:

In this, and later figures herein, we depict the dative bond from nitrogen to metal (here, Ir) as a straight line.

As used herein, the term “organic” includes polymeric materials as well as small molecule organic materials that may be used to fabricate organic opto-electronic devices. “Small molecule” refers to any organic material that is not a polymer, and “small molecules” may actually be quite large. Small molecules may include repeat units in some circumstances. For example, using a long chain alkyl group as a substituent does not remove a molecule from the “small molecule” class. Small molecules may also be incorporated into polymers, for example as a pendent group on a polymer backbone or as a part of the backbone. Small molecules may also serve as the core moiety of a dendrimer, which consists of a series of chemical shells built on the core moiety. The core moiety of a dendrimer may be a fluorescent or phosphorescent small molecule emitter. A dendrimer may be a “small molecule,” and it is believed that all dendrimers currently used in the field of OLEDs are small molecules.

As used herein, “top” means furthest away from the substrate, while “bottom” means closest to the substrate. Where a first layer is described as “disposed over” a second layer, the first layer is disposed further away from substrate. There may be other layers between the first and second layer, unless it is specified that the first layer is “in contact with” the second layer. For example, a cathode may be described as “disposed over” an anode, even though there are various organic layers in between.

As used herein, “solution processible” means capable of being dissolved, dispersed, or transported in and/or deposited from a liquid medium, either in solution or suspension form.

A ligand may be referred to as “photoactive” when it is believed that the ligand directly contributes to the photoactive properties of an emissive material. A ligand may be referred to as “ancillary” when it is believed that the ligand does not contribute to the photoactive properties of an emissive material, although an ancillary ligand may alter the properties of a photoactive ligand.

As used herein, and as would be generally understood by one skilled in the art, a first “Highest Occupied Molecular Orbital” (HOMO) or “Lowest Unoccupied Molecular Orbital” (LUMO) energy level is “greater than” or “higher than” a second HOMO or LUMO energy level if the first energy level is closer to the vacuum energy level. Since ionization potentials (IP) are measured as a negative energy relative to a vacuum level, a higher HOMO energy level corresponds to an IP having a smaller absolute value (an IP that is less negative). Similarly, a higher LUMO energy level corresponds to an electron affinity (EA) having a smaller absolute value (an EA that is less negative). On a conventional energy level diagram, with the vacuum level at the top, the LUMO energy level of a material is higher than the HOMO energy level of the same material. A “higher” HOMO or LUMO energy level appears closer to the top of such a diagram than a “lower” HOMO or LUMO energy level.

As used herein, and as would be generally understood by one skilled in the art, a first work function is “greater than” or “higher than” a second work function if the first work function has a higher absolute value. Because work functions are generally measured as negative numbers relative to vacuum level, this means that a “higher” work function is more negative. On a conventional energy level diagram, with the vacuum level at the top, a “higher” work function is illustrated as further away from the vacuum level in the downward direction. Thus, the definitions of HOMO and LUMO energy levels follow a different convention than work functions.

More details on OLEDs, and the definitions described above, can be found in U.S. Pat. No. 7,279,704, which is incorporated herein by reference in its entirety.

SUMMARY

According to an embodiment, a compound having the formula Os(L)_(n) is disclosed; wherein Os is Osmium (IV) metal, L is a ligand coordinating to the Os atom, and n is an integer from 1 to 6; wherein each L can be same or different; wherein at least one L is a multidentate ligand; and wherein the compound is neutral.

According to another aspect, a first device comprising a first organic light emitting device is provided. The first organic light emitting device can comprise an anode, a cathode, and an organic layer, disposed between the anode and the cathode. The organic layer can comprise a compound having the formula Os(L)_(n), wherein Os is Osmium (IV) metal, L is a ligand coordinating to the Os atom, and n is an integer from 1 to 6, wherein each L can be same or different, wherein at least one L is a multidentate ligand, and wherein the compound is neutral. The first device can be a consumer product, an organic light-emitting device, and/or a lighting panel.

Many osmium(II) based phosphorescent emitters for OLED applications are known. However, because of the shallow HOMO level of Os(II) resulting in a small band gap, tuning the color of the emission of osmium(II) based emitters to blue region can be difficult. The inventors have found that osmium(IV) complexes have deeper HOMO than Os(II) and therefore, it is easier to achieve blue emission. Compared to Os(II) based emitters, Os(IV) complexes have deeper HOMO which enables the incorporation of main stream organic hosts; i.e. a host with HOMO level around −5.39 eV and LUMO level around −1.21 eV. Also, because of the deeper HOMO, OLEDs utilizing Os(IV) based emitters are more immune from formation of exciplex in the device. Os(IV) complexes in general is more difficult to oxidize than Os(II) complexes because of the higher oxidation state; therefore; the band gap is potentially wider than Os(II) and easier to achieve blue emission than Os(II).

Since Os(II) complexes in general has a very shallow HOMO level. It's very difficult to tune emission energy to blue region due to the small band gap. On the other hand, HOMO level of Os(IV) complexes are deeper and potentially easier to reach blue region.

Osmium(II) complexes have been investigated for OLED applications. The octahedral ligand arrangement of the Os(II) complexes resembles that of Ir(III) complexes. Os(II) complexes generally exhibit low oxidation potential, i.e. shallow HOMO energy level than Ir(III) complexes. Os(IV) complexes offers a great deal of flexibility for color tuning and bring the HOMO level to proper alignment in the device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an organic light emitting device.

FIG. 2 shows an inverted organic light emitting device that does not have a separate electron transport layer.

FIG. 3 shows a Molecular structure of complex trihydride.

DETAILED DESCRIPTION

Generally, an OLED comprises at least one organic layer disposed between and electrically connected to an anode and a cathode. When a current is applied, the anode injects holes and the cathode injects electrons into the organic layer(s). The injected holes and electrons each migrate toward the oppositely charged electrode. When an electron and hole localize on the same molecule, an “exciton,” which is a localized electron-hole pair having an excited energy state, is formed. Light is emitted when the exciton relaxes via a photoemissive mechanism. In some cases, the exciton may be localized on an excimer or an exciplex. Non-radiative mechanisms, such as thermal relaxation, may also occur, but are generally considered undesirable.

The initial OLEDs used emissive molecules that emitted light from their singlet states (“fluorescence”) as disclosed, for example, in U.S. Pat. No. 4,769,292, which is incorporated by reference in its entirety. Fluorescent emission generally occurs in a time frame of less than 10 nanoseconds.

More recently, OLEDs having emissive materials that emit light from triplet states (“phosphorescence”) have been demonstrated. Baldo et al., “Highly Efficient Phosphorescent Emission from Organic Electroluminescent Devices,” Nature, vol. 395, 151-154, 1998; (“Baldo-I”) and Baldo et al., “Very high-efficiency green organic light-emitting devices based on electrophosphorescence,” Appl. Phys. Lett., vol. 75, No. 3, 4-6 (1999) (“Baldo-II”), which are incorporated by reference in their entireties. Phosphorescence is described in more detail in U.S. Pat. No. 7,279,704 at cols. 5-6, which are incorporated by reference.

FIG. 1 shows an organic light emitting device 100. The figures are not necessarily drawn to scale. Device 100 may include a substrate 110, an anode 115, a hole injection layer 120, a hole transport layer 125, an electron blocking layer 130, an emissive layer 135, a hole blocking layer 140, an electron transport layer 145, an electron injection layer 150, a protective layer 155, a cathode 160, and a barrier layer 170. Cathode 160 is a compound cathode having a first conductive layer 162 and a second conductive layer 164. Device 100 may be fabricated by depositing the layers described, in order. The properties and functions of these various layers, as well as example materials, are described in more detail in U.S. Pat. No. 7,279,704 at cols. 6-10, which are incorporated by reference.

More examples for each of these layers are available. For example, a flexible and transparent substrate-anode combination is disclosed in U.S. Pat. No. 5,844,363, which is incorporated by reference in its entirety. An example of a p-doped hole transport layer is m-MTDATA doped with F4-TCNQ at a molar ratio of 50:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. Examples of emissive and host materials are disclosed in U.S. Pat. No. 6,303,238 to Thompson et al., which is incorporated by reference in its entirety. An example of an n-doped electron transport layer is BPhen doped with Li at a molar ratio of 1:1, as disclosed in U.S. Patent Application Publication No. 2003/0230980, which is incorporated by reference in its entirety. U.S. Pat. Nos. 5,703,436 and 5,707,745, which are incorporated by reference in their entireties, disclose examples of cathodes including compound cathodes having a thin layer of metal such as Mg:Ag with an overlying transparent, electrically-conductive, sputter-deposited ITO layer. The theory and use of blocking layers is described in more detail in U.S. Pat. No. 6,097,147 and U.S. Patent Application Publication No. 2003/0230980, which are incorporated by reference in their entireties. Examples of injection layers are provided in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety. A description of protective layers may be found in U.S. Patent Application Publication No. 2004/0174116, which is incorporated by reference in its entirety.

FIG. 2 shows an inverted OLED 200. The device includes a substrate 210, a cathode 215, an emissive layer 220, a hole transport layer 225, and an anode 230. Device 200 may be fabricated by depositing the layers described, in order. Because the most common OLED configuration has a cathode disposed over the anode, and device 200 has cathode 215 disposed under anode 230, device 200 may be referred to as an “inverted” OLED. Materials similar to those described with respect to device 100 may be used in the corresponding layers of device 200. FIG. 2 provides one example of how some layers may be omitted from the structure of device 100.

The simple layered structure illustrated in FIGS. 1 and 2 is provided by way of non-limiting example, and it is understood that embodiments of the invention may be used in connection with a wide variety of other structures. The specific materials and structures described are exemplary in nature, and other materials and structures may be used. Functional OLEDs may be achieved by combining the various layers described in different ways, or layers may be omitted entirely, based on design, performance, and cost factors. Other layers not specifically described may also be included. Materials other than those specifically described may be used. Although many of the examples provided herein describe various layers as comprising a single material, it is understood that combinations of materials, such as a mixture of host and dopant, or more generally a mixture, may be used. Also, the layers may have various sublayers. The names given to the various layers herein are not intended to be strictly limiting. For example, in device 200, hole transport layer 225 transports holes and injects holes into emissive layer 220, and may be described as a hole transport layer or a hole injection layer. In one embodiment, an OLED may be described as having an “organic layer” disposed between a cathode and an anode. This organic layer may comprise a single layer, or may further comprise multiple layers of different organic materials as described, for example, with respect to FIGS. 1 and 2.

Structures and materials not specifically described may also be used, such as OLEDs comprised of polymeric materials (PLEDs) such as disclosed in U.S. Pat. No. 5,247,190 to Friend et al., which is incorporated by reference in its entirety. By way of further example, OLEDs having a single organic layer may be used. OLEDs may be stacked, for example as described in U.S. Pat. No. 5,707,745 to Forrest et al, which is incorporated by reference in its entirety. The OLED structure may deviate from the simple layered structure illustrated in FIGS. 1 and 2. For example, the substrate may include an angled reflective surface to improve out-coupling, such as a mesa structure as described in U.S. Pat. No. 6,091,195 to Forrest et al., and/or a pit structure as described in U.S. Pat. No. 5,834,893 to Bulovic et al., which are incorporated by reference in their entireties.

Unless otherwise specified, any of the layers of the various embodiments may be deposited by any suitable method. For the organic layers, preferred methods include thermal evaporation, ink-jet, such as described in U.S. Pat. Nos. 6,013,982 and 6,087,196, which are incorporated by reference in their entireties, organic vapor phase deposition (OVPD), such as described in U.S. Pat. No. 6,337,102 to Forrest et al., which is incorporated by reference in its entirety, and deposition by organic vapor jet printing (OVJP), such as described in U.S. Pat. No. 7,431,968, which is incorporated by reference in its entirety. Other suitable deposition methods include spin coating and other solution based processes. Solution based processes are preferably carried out in nitrogen or an inert atmosphere. For the other layers, preferred methods include thermal evaporation. Preferred patterning methods include deposition through a mask, cold welding such as described in U.S. Pat. Nos. 6,294,398 and 6,468,819, which are incorporated by reference in their entireties, and patterning associated with some of the deposition methods such as ink-jet and OVJD. Other methods may also be used. The materials to be deposited may be modified to make them compatible with a particular deposition method. For example, substituents such as alkyl and aryl groups, branched or unbranched, and preferably containing at least 3 carbons, may be used in small molecules to enhance their ability to undergo solution processing. Substituents having 20 carbons or more may be used, and 3-20 carbons is a preferred range. Materials with asymmetric structures may have better solution processibility than those having symmetric structures, because asymmetric materials may have a lower tendency to recrystallize. Dendrimer substituents may be used to enhance the ability of small molecules to undergo solution processing.

Devices fabricated in accordance with embodiments of the present invention may further optionally comprise a barrier layer. One purpose of the barrier layer is to protect the electrodes and organic layers from damaging exposure to harmful species in the environment including moisture, vapor and/or gases, etc. The barrier layer may be deposited over, under or next to a substrate, an electrode, or over any other parts of a device including an edge. The barrier layer may comprise a single layer, or multiple layers. The barrier layer may be formed by various known chemical vapor deposition techniques and may include compositions having a single phase as well as compositions having multiple phases. Any suitable material or combination of materials may be used for the barrier layer. The barrier layer may incorporate an inorganic or an organic compound or both. The preferred barrier layer comprises a mixture of a polymeric material and a non-polymeric material as described in U.S. Pat. No. 7,968,146, PCT Pat. Application Nos. PCT/US2007/023098 and PCT/US2009/042829, which are herein incorporated by reference in their entireties. To be considered a “mixture”, the aforesaid polymeric and non-polymeric materials comprising the barrier layer should be deposited under the same reaction conditions and/or at the same time. The weight ratio of polymeric to non-polymeric material may be in the range of 95:5 to 5:95. The polymeric material and the non-polymeric material may be created from the same precursor material. In one example, the mixture of a polymeric material and a non-polymeric material consists essentially of polymeric silicon and inorganic silicon.

Devices fabricated in accordance with embodiments of the invention may be incorporated into a wide variety of consumer products, including flat panel displays, computer monitors, medical monitors, televisions, billboards, lights for interior or exterior illumination and/or signaling, heads up displays, fully transparent displays, flexible displays, laser printers, telephones, cell phones, personal digital assistants (PDAs), laptop computers, digital cameras, camcorders, viewfinders, micro-displays, 3-D displays, vehicles, a large area wall, theater or stadium screen, or a sign. Various control mechanisms may be used to control devices fabricated in accordance with the present invention, including passive matrix and active matrix. Many of the devices are intended for use in a temperature range comfortable to humans, such as 18 degrees C. to 30 degrees C., and more preferably at room temperature (20-25 degrees C.), but could be used outside this temperature range, for example, from −40 degree C. to +80 degree C.

The materials and structures described herein may have applications in devices other than OLEDs. For example, other optoelectronic devices such as organic solar cells and organic photodetectors may employ the materials and structures. More generally, organic devices, such as organic transistors, may employ the materials and structures.

The terms halo, halogen, alkyl, cycloalkyl, alkenyl, alkynyl, aralkyl, heterocyclic group, aryl, aromatic group, and heteroaryl are known to the art, and are defined in U.S. Pat. No. 7,279,704 at cols. 31-32, which are incorporated herein by reference.

As used herein, “substituted” indicates that a substituent other than H is bonded to the relevant carbon. Thus, where R² is monosubstituted, then one R² must be other than H. Similarly, where R³ is disubstituted, then two of R³ must be other than H. Similarly, where R² is unsubstituted R² is hydrogen for all available positions.

According to an aspect of the present disclosure, a novel compound which is an osmium(IV) complex having the formula Os(L)_(n) is disclosed; wherein Os is osmium(IV) metal, L is a ligand coordinating to the Os atom, and n is an integer from 1 to 6; wherein each L can be same or different; wherein at least one L is a multidentate ligand; and wherein the compound is neutral.

In one embodiment, the multidentate ligand L of the Os(IV) complex is selected from the group AA consisting of:

wherein X is a neutral coordinating atom selected from the group consisting of carbene, phosphorus, and nitrogen;

wherein C is an anionic coordinating carbon atom;

wherein N is an anionic coordinating nitrogen atom;

wherein O is an anionic coordinating oxygen atom; and

In one embodiment of the group AA, the neutral carbene is N-heterocyclic carbene; the neutral phosphorus is phosphorus atom of a trisubstituted phosphine; and the neutral nitrogen is sp² nitrogen atom of N-heterocyclic ring selected from the group consisting of pyridine, imidazole, benzoimidazole, pyrazole, and triazole. The anionic coordinating carbon can be sp² carbon atom selected from the group consisting of benzene, pyridine, furan, thiophene, and pyrrole. The anionic coordinating nitrogen can be sp² nitrogen atom of N-heterocyclic ring selected from the group consisting of imidazole, benzoimidazole, pyrazole, and triazole. The anionic oxygen atom can be oxygen atom from carboxylic acid or ether.

In another embodiment of the osmium(IV) complex having the formula Os(L)n, wherein the multidentate ligand L is selected from the group AA, n is 2 and each L is a tridentate ligand. In such embodiment, the osmium(IV) complex compound can have a formula selected from the group consisting of:

Where the osmium(IV) complex having the formula Os(L)_(n), wherein the multidentate ligand L is selected from the group AA, and n is 2 and each L is a tridentate ligand, the osmium(IV) complex compound can be selected from the group consisting of:

In another embodiment, the osmium(IV) complex compound has the formula Os(L)_(n), wherein the multidentate ligand L is selected from the group AA, and n is 3 and each L is a bidentate ligand. In such embodiment, the osmium(IV) complex compound can have a formula selected from the group consisting of:

Where the osmium(IV) complex is a compound having the formula Os(L)_(n) where the multidentate ligand L is selected from the group AA, and n is 3 and each L is a bidentate ligand, the osmium(IV) complex compound can be selected from the group consisting of:

In one embodiment, the osmium(IV) complex is a compound having the formula Os(L)_(n), wherein the multidentate ligand L is selected from the group AA, and wherein n is 3 and one L is a tridentate ligand and two Ls are bidentate ligand. In such embodiment, the osmium(IV) complex compound can have a formula selected from the group consisting of:

Where the osmium(IV) complex is a compound having the formula Os(L)_(n), wherein the multidentate ligand L is selected from the group AA, and n is 3 and one L is a tridentate ligand and two Ls are bidentate ligand, the compound can be selected from the group consisting of:

In one embodiment, the osmium(IV) complex is a compound having the formula Os(L)_(n), wherein the multidentate ligand L is selected from the group AA, and wherein one L is a tetradentate ligand. In such embodiment, the compound can have a formula selected from the group consisting of:

In the embodiment where the osmium(IV) complex is a compound having the formula Os(L)_(n), wherein the multidentate ligand L is selected from the group AA, and one L is a tridentate ligand, the compound can be selected from the group consisting of:

In one embodiment, the osmium(IV) complex compound having the formula Os(L)_(n), wherein the multidentate ligand L is selected from the group AA, has a formula

wherein H is hydride. In such embodiment, the compound can be selected from the group consisting of:

According to another aspect, a first device comprising a first organic light emitting device is provided. The first organic light emitting device can comprise an anode, a cathode, and an organic layer, disposed between the anode and the cathode. The organic layer can comprise a compound having the formula Os(L)_(n), wherein Os is Osmium(IV) metal, L is a ligand coordinating to the Os atom, and n is an integer from 1 to 6, wherein each L can be same or different, wherein at least one L is a multidentate ligand, and wherein the compound is neutral. The first device can be a consumer product, an organic light-emitting device, and/or a lighting panel.

In one embodiment of the first device, the organic layer is an emissive layer and the compound having the formula Os(L)_(n) is an emissive dopant. In another embodiment, the organic layer is an emissive layer and the compound having the formula OS(L)_(n) is a non-emissive dopant.

In another embodiment of the first device, the organic layer further comprises a host material. The host material can comprise a triphenylene containing benzo-fused thiophene or benzo-fused furan; wherein any substituent in the host material is an unfused substituent independently selected from the group consisting of C_(n)H_(2n+1), OC_(n)H_(2n+1), OAr₁, N(C_(n)H_(2n+1))₂, N(Ar₁)(Ar₂), CH═CH—C_(n)H_(2n+1), C≡C—C_(n)H_(2n+1), Ar₁, Ar₁—Ar₂, C_(n)H_(2n)—Ar₁, or no substitution;

wherein n is from 1 to 10; and

wherein Ar₁ and Ar₂ are independently selected from the group consisting of benzene, biphenyl, naphthalene, triphenylene, carbazole, and heteroaromatic analogs thereof.

In another embodiment, the host material comprises at least one chemical group selected from the group consisting of carbazole, dibenzothiphene, dibenzofuran, dibenzoselenophene, azacarbazole, aza-dibenzothiophene, aza-dibenzofuran, and aza-dibenzoselenophene.

In another embodiment, the host material is selected from the group consisting of

and combinations thereof.

In yet another embodiment, the host material comprises a metal complex.

According to another aspect of the present disclosure, a novel formulation is disclosed. The formulation comprises a compound which is an osmium(IV) complex having the formula Os(L)n is disclosed; wherein Os is osmium(IV) metal, L is a ligand coordinating to the Os atom, and n is an integer from 1 to 6; wherein each L can be same or different; wherein at least one L is a multidentate ligand; and wherein the compound is neutral.

Combination with Other Materials

The materials described herein as useful for a particular layer in an organic light emitting device may be used in combination with a wide variety of other materials present in the device. For example, emissive dopants disclosed herein may be used in conjunction with a wide variety of hosts, transport layers, blocking layers, injection layers, electrodes and other layers that may be present. The materials described or referred to below are non-limiting examples of materials that may be useful in combination with the compounds disclosed herein, and one of skill in the art can readily consult the literature to identify other materials that may be useful in combination.

HIL/HTL:

A hole injecting/transporting material to be used in the present invention is not particularly limited, and any compound may be used as long as the compound is typically used as a hole injecting/transporting material. Examples of the material include, but not limit to: a phthalocyanine or porphryin derivative; an aromatic amine derivative; an indolocarbazole derivative; a polymer containing fluorohydrocarbon; a polymer with conductivity dopants; a conducting polymer, such as PEDOT/PSS; a self-assembly monomer derived from compounds such as phosphonic acid and sliane derivatives; a metal oxide derivative, such as MoO_(x); a p-type semiconducting organic compound, such as 1,4,5,8,9,12-Hexaazatriphenylenehexacarbonitrile; a metal complex, and a cross-linkable compounds.

Examples of aromatic amine derivatives used in HIL or HTL include, but not limit to the following general structures:

Each of Ar¹ to Ar⁹ is selected from the group consisting aromatic hydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl, triphenylene, naphthalene, anthracene, phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, azulene; group consisting aromatic heterocyclic compounds such as dibenzothiophene, dibenzofuran, dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene, benzoselenophene, carbazole, indolocarbazole, pyridylindole, pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole, oxadiazole, oxatriazole, dioxazole, thiadiazole, pyridine, pyridazine, pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine, indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole, benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline, quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine, phenazine, phenothiazine, phenoxazine, benzofuropyridine, furodipyridine, benzothienopyridine, thienodipyridine, benzoselenophenopyridine, and selenophenodipyridine; and group consisting 2 to 10 cyclic structural units which are groups of the same type or different types selected from the aromatic hydrocarbon cyclic group and the aromatic heterocyclic group and are bonded to each other directly or via at least one of oxygen atom, nitrogen atom, sulfur atom, silicon atom, phosphorus atom, boron atom, chain structural unit and the aliphatic cyclic group. Wherein each Ar is further substituted by a substituent selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof.

In one aspect, Ar¹ to Ar⁹ is independently selected from the group consisting of:

wherein k is an integer from 1 to 20; X¹⁰¹ to X¹⁰⁸ is C (including CH) or N; Z¹⁰¹ is NAr¹, O, or S; Ar¹ has the same group defined above.

Examples of metal complexes used in HIL or HTL include, but not limit to the following general formula:

wherein Met is a metal, which can have an atomic weight greater than 40; (Y¹⁰¹-Y¹⁰²) is a bidentate ligand, Y¹⁰¹ and Y¹⁰² are independently selected from C, N, O, P, and S; L¹⁰¹ is an ancillary ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal; and k′+k″ is the maximum number of ligands that may be attached to the metal.

In one aspect, (Y¹⁰¹-Y¹⁰²) is a 2-phenylpyridine derivative. In another aspect, (Y¹⁰¹-Y¹⁰²) is a carbene ligand. In another aspect, Met is selected from Ir, Pt, Os, and Zn. In a further aspect, the metal complex has a smallest oxidation potential in solution vs. Fc⁺/Fc couple less than about 0.6 V.

Host:

The light emitting layer of the organic EL device of the present invention preferably contains at least a metal complex as light emitting material, and may contain a host material using the metal complex as a dopant material. Examples of the host material are not particularly limited, and any metal complexes or organic compounds may be used as long as the triplet energy of the host is larger than that of the dopant. While the Table below categorizes host materials as preferred for devices that emit various colors, any host material may be used with any dopant so long as the triplet criteria is satisfied.

Examples of metal complexes used as host are preferred to have the following general formula:

wherein Met is a metal; (Y¹⁰³-Y¹⁰⁴) is a bidentate ligand, Y¹⁰³ and Y¹⁰⁴ are independently selected from C, N, O, P, and S; L¹⁰¹ is an another ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal; and k′+k″ is the maximum number of ligands that may be attached to the metal.

In one aspect, the metal complexes are:

wherein (O—N) is a bidentate ligand, having metal coordinated to atoms O and N.

In another aspect, Met is selected from Ir and Pt. In a further aspect, (Y¹⁰³-Y¹⁰⁴) is a carbene ligand.

Examples of organic compounds used as host are selected from the group consisting aromatic hydrocarbon cyclic compounds such as benzene, biphenyl, triphenyl, triphenylene, naphthalene, anthracene, phenalene, phenanthrene, fluorene, pyrene, chrysene, perylene, azulene; group consisting aromatic heterocyclic compounds such as dibenzothiophene, dibenzofuran, dibenzoselenophene, furan, thiophene, benzofuran, benzothiophene, benzoselenophene, carbazole, indolocarbazole, pyridylindole, pyrrolodipyridine, pyrazole, imidazole, triazole, oxazole, thiazole, oxadiazole, oxatriazole, dioxazole, thiadiazole, pyridine, pyridazine, pyrimidine, pyrazine, triazine, oxazine, oxathiazine, oxadiazine, indole, benzimidazole, indazole, indoxazine, benzoxazole, benzisoxazole, benzothiazole, quinoline, isoquinoline, cinnoline, quinazoline, quinoxaline, naphthyridine, phthalazine, pteridine, xanthene, acridine, phenazine, phenothiazine, phenoxazine, benzofuropyridine, furodipyridine, benzothienopyridine, thienodipyridine, benzoselenophenopyridine, and selenophenodipyridine; and group consisting 2 to 10 cyclic structural units which are groups of the same type or different types selected from the aromatic hydrocarbon cyclic group and the aromatic heterocyclic group and are bonded to each other directly or via at least one of oxygen atom, nitrogen atome, sulfur atom, silicon atom, phosphorus atom, boron atom, chain structural unit and the aliphatic cyclic group. Wherein each group is further substituted by a substituent selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof.

In one aspect, host compound contains at least one of the following groups in the molecule:

wherein R¹⁰¹ to R¹⁰⁷ is independently selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof, when it is aryl or heteroaryl, it has the similar definition as Ar's mentioned above. k is an integer from 0 to 20 or 1 to 20; k′″ is an integer from 0 to 20. X¹⁰¹ to X¹⁰⁸ is selected from C (including CH) or N. Z¹⁰¹ and Z¹⁰² is selected from NR¹⁰¹, O, or S.

HBL:

A hole blocking layer (HBL) may be used to reduce the number of holes and/or excitons that leave the emissive layer. The presence of such a blocking layer in a device may result in substantially higher efficiencies as compared to a similar device lacking a blocking layer. Also, a blocking layer may be used to confine emission to a desired region of an OLED.

In one aspect, compound used in HBL contains the same molecule or the same functional groups used as host described above.

In another aspect, compound used in HBL contains at least one of the following groups in the molecule:

wherein k is an integer from 1 to 20; L¹⁰¹ is an another ligand, k′ is an integer from 1 to 3.

ETL:

Electron transport layer (ETL) may include a material capable of transporting electrons. Electron transport layer may be intrinsic (undoped), or doped. Doping may be used to enhance conductivity. Examples of the ETL material are not particularly limited, and any metal complexes or organic compounds may be used as long as they are typically used to transport electrons.

In one aspect, compound used in ETL contains at least one of the following groups in the molecule:

wherein R¹⁰¹ is selected from the group consisting of hydrogen, deuterium, halide, alkyl, cycloalkyl, heteroalkyl, arylalkyl, alkoxy, aryloxy, amino, silyl, alkenyl, cycloalkenyl, heteroalkenyl, alkynyl, aryl, heteroaryl, acyl, carbonyl, carboxylic acids, ester, nitrile, isonitrile, sulfanyl, sulfinyl, sulfonyl, phosphino, and combinations thereof, when it is aryl or heteroaryl, it has the similar definition as Ar's mentioned above. Ar¹ to Ar³ has the similar definition as Ar's mentioned above. k is an integer from 1 to 20. X¹⁰¹ to X¹⁰⁸ is selected from C (including CH) or N.

In another aspect, the metal complexes used in ETL contains, but not limit to the following general formula:

wherein (O—N) or (N—N) is a bidentate ligand, having metal coordinated to atoms O, N or N, N; L¹⁰¹ is another ligand; k′ is an integer value from 1 to the maximum number of ligands that may be attached to the metal.

In any above-mentioned compounds used in each layer of the OLED device, the hydrogen atoms can be partially or fully deuterated. Thus, any specifically listed substituent, such as, without limitation, methyl, phenyl, pyridyl, etc. encompasses undeuterated, partially deuterated, and fully deuterated versions thereof. Similarly, classes of substituents such as, without limitation, alkyl, aryl, cycloalkyl, heteroaryl, etc. also encompass undeuterated, partially deuterated, and fully deuterated versions thereof.

In addition to and/or in combination with the materials disclosed herein, many hole injection materials, hole transporting materials, host materials, dopant materials, exiton/hole blocking layer materials, electron transporting and electron injecting materials may be used in an OLED. Non-limiting examples of the materials that may be used in an OLED in combination with materials disclosed herein are listed in Table 1 below. Table 1 lists non-limiting classes of materials, non-limiting examples of compounds for each class, and references that disclose the materials.

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EXPERIMENTAL VERIFICATION

All reactions were carried out with rigorous exclusion of air using Schlenk-tube techniques. Solvents, except acetonitrile that was dried and distilled under argon, were obtained oxygen-free and water-free from an MBraun solvent purification apparatus. ¹H, ³¹P{¹H}, ¹⁹F and ¹³C{¹H} NMR spectra were recorded on Bruker 300 ARX, Bruker Avance 300 MHz, and Bruker Avance 400 MHz instruments. Chemical shifts, expressed in parts per million, are referenced to residual solvent peaks (¹H, ¹³C{¹H}) or external 85% H₃PO₄ (³¹P{¹H}), or external CFCl₃ (¹⁹F). Coupling constants J and N are given in hertz. Attenuated total reflection infrared spectra (ATR-IR) of solid samples were run on a Perkin-Elmer Spectrum 100 FT-IR spectrometer. C, H, and N analyses were carried out in a Perkin-Elmer 2400 CHNS/O analyzer. High-resolution electrospray mass spectra were acquired using a MicroTOF-Q hybrid quadrupole time-of-flight spectrometer (Bruker Daltonics, Bremen, Germany). OsH₆(P^(i)Pr₃)₂ was prepared by published method. (Inorganic Chemistry 1991, 30, 288-293)

Preparation of Trihydride-Bi2 (Compound V-1):

Toluene (40 mL) and NEt₃ (120 μl, 0.86 mmol) were added to a mixture of OsH₆(P^(i)Pr₃)₂ (400 mg, 0.78 mmol) and 1-phenyl-3-methyl-1-H-benzimidazolium tetrafluoroborate (Bi2, 229.2 mg, 0.78 mmol). The resulting mixture was refluxed for 4 hours and then the brownish-yellow solution was extracted and concentrated under vacuum to 1 mL. MeOH was added (8 mL), affording a white powder which was filtered and washed with MeOH (2×4 mL) at 195 K and dried in vacuo. Yield: 76.4% 426.0 mg. Anal. Calcd. for C₃₂H₅₆N₂OsP₂: C, 53.31%; H, 7.83%; N, 3.89%. Found: C, 53.01%; H, 8.14%; N, 3.91%. HRMS (electrospray, m/z): calcd for C₃₂H₅₁N₂OsP₂[M-5H]⁺: 719.3295; found: 719.3281. IR (cm⁻¹): υ(Os—H) 2044 (m).

¹H NMR (400 MHz, C₆D₆, 293 K): δ 8.66 (d, J_(H-H)=7.2, 1H, CH Ph), 7.91-7.88 (m, 2H, CH Ph, CH Bzm), 7.24 (ddd, =7.2, J_(H-H)=7.2, J_(H-H)=1.0, 1H, CH Ph), 7.10 (ddd, =7.2, J_(H-H)=7.2, J_(H-H)=1.0, 1H, CH Ph), 7.05-6.99 (m, 2H, CH Bzm), 6.91 (m, 1H, CH Bzm), 3.92 (s, 3H, N—CH₃), 1.79 (m, 6H, PCH), 0.96 (dvt, N=12.5, J=6.9, 18H, PCH(CH₃)₂) 0.83 (dvt, N=12.5, J=6.9, 18H, PCH(CH₃)₂), −8.18 (br, 1H, Os—H), −9.94 (br, 2H, Os—H). T_(1(min)) (ms, OsH, 400 MHz, CD₂Cl₂): 118.2±10 (213 K). ¹³C{¹H}-APT NMR, HMBC and HSQC (100.6 MHz, C₆D₆, 293 K): δ 206.3 (t, J_(C-P)=5.4, NCN Bzm), 157.9 (t, J_(C-P)=6.7, Os—C Ph), 148.9 (s, N—C Ph), 148.0 (s, C—HPh), 137.5 (s, C Bzm), 133.6 (s, C Bzm), 124.3 (s, C—H Ph), 122.1 (s, C-HBzm), 121.3 (s, C-HBzm), 119.8 (s, C—HPh), 112.5 (s, C—H Bzm), 110.4 (s, C—H Ph), 109.1 (s, C—H Bzm), 36.6 (s, N—CH₃ Bzm), 27.9 (dvt, N=25.0, P—CH), 19.9 (s, PCH(CH₃)₂), 19.8 (s, PCH(CH₃)₂). ³¹P{¹H} NMR (162.0 MHz, C₆D₆, 293 K): δ 25.7 (s). FIG. 3 shows X-ray diffraction analysis of this complex, showing the molecular structure of the complex trihydride. Selected bond lengths (Å) and angles (°): Os—P(1)=2.3435(7), Os—P(2)=2.3538(7), Os—C(1)=2.069(2), Os—C(10)=2.140(3), H(01)-H(02)=1.69(3), H(01)-H(03)=1.74(3); P(1)-Os—P(2)=165.17(2), C(1)-Os—C(10)=75.69(9).

Preparation of Trihydride-Bi3 (Compound V-2):

Toluene (40 mL) and NEt₃ (120 μl, 0.86 mmol) were added to a mixture of OsH₆(P^(i)Pr₃)₂ (400 mg, 0.78 mmol) and 1-phenyl-3-methyl-1-H,-5,6-dimethyl-benzimidazolium tetrafluoroborate (Bi3, 250.9 mg, 0.78 mmol). The resulting mixture was refluxed for 4 hour and then the solution was extracted and concentrated under vacuum to 1 mL. MeOH was added (8 mL) and the resulting white powder was filtered and washed with MeOH (3×3 mL). Yield: 84% 490.7. Anal. Calcd. for C₃₄H₆₀N₂OsP₂.CH₃OH: C 53.82%; H 8.26%; N 3.59%. Found: C, 53.99%; H, 8.51%; N, 3.67%. HRMS (electrospray, m/z): calcd for C₃₄H₅₅OsP₂N₂ [M-H]⁺: 745.3452; found: 745.3460. IR: (cm⁻¹) υ(Os—H) 2049 (m), υ(Os—H) 2027 (m).

¹H NMR (400 MHz, C₆D₆, 293 K): δ 8.67 (d, =7.5, 1H, CH Ph), 8.02 (dd, J_(H-H)=7.5, J_(H-H)=0.8, 1H, CH Ph), 7.88 (s, 1H, H Bzm), 7.24 (ddd, J_(H-H)=7.5, J_(H-H)=7.5, J_(H-H)=1.1, 1H, CH Ph), 7.10 (ddd, J_(H-H)=7.5, J_(H-H)=7.5, J_(H-H)=0.8, 1H, CH Ph), 6.80 (s, 1H, CH Bzm), 3.96 (s, 3H, N—CH₃), 2.13 and 2.12 (both s, each 3H, —CH₃ Bzm), 1.84 (m, 6H, P—CH), 0.99 (dvt, N=12.5, J=6.9, 18H, PCH(CH₃)₂), 0.87 (dvt, N=12.5, J=6.9, 18H, PCH(CH₃)₂), −8.21 (br, 1H, Os—H), −9.96 (br, 2H, Os—H). T_(1(min)) (ms, OsH, 400 MHz, CD₂Cl₂): 115.0±11 (233 K). ¹³C{¹H}-APT NMR, HMBC and HSQC (100.6 MHz, C₆D₆, 293 K): δ 204.9 (t, J_(C-P)=5.6, NCN Bzm), 157.7 (t, J_(C-P)=6.7, Os—C Ph), 149.2 (s, N—C Ph), 148.0 (s, C—H Ph), 136.2 (s, CBzm), 132.3 (s, CBzm), 130.3 (s, C Bzm), 129.6 (s, C Bzm), 124.2 (s, C—H Ph), 119.8 (s, C—H Ph), 112.3 (s, C—H Ph), 111.6 (s, C—H Bzm), 110.2 (s, C—H Bzm), 36.8 (s, NCH3), 28.1 (dvt, N=24.8, P—CH), 20.2 and 20.1 (both s, −CH₃Bzm), 20.0 (s, PCH(CH₃)₂), 19.8 (s, PCH(CH₃)₂). ³¹P{¹H} NMR (162.0 MHz, C₆D₆, 293 K): δ 25.6 (s).

Table 2 below shows HOMO and LUMO levels for the Osmium complexes calculated by DFT (density function theory) calculation using Gaussian/B31yp/cep-31g.

TABLE 2 Compound HOMO LUMO T1 Number Structure (eV) (eV) (nm) Comparative Compound 1 −4.30 −0.95 497 Comparative Compound 2 −4.52 −1.96 929 Compound IV-3 −4.98 −2.50 1296 Compound I-1

−5.12 −2.82 974 Compound IV-5

−5.01 −1.59 575 Compound IV-4

−4.91 −1.29 493 Compound III-2

−5.06 −1.17 425 Compound III-1

−5.02 −1.37 458 Compound IV-1

−5.02 −1.15 418 Compound III-4

−5.01 −1.25 433 Compound III-5

−5.00 −1.38 466

Osmium (II) complexes have been investigated for OLED applications. (see review: Eur. J. Inorg. Chem. 2006, 3319-3332). The octahedral ligand arrangement of the Os(II) complexes resembles that of Ir(III) complexes. Os(II) complexes generally exhibit low oxidation potential, i.e. shallow HOMO energy level than Ir(III) complexes. The extremely shallow HOMO level of Osmium bis(tridenate) complexes make it difficult to fit in the main stream OLED device structure. According to the invention disclosed in the present disclosure, by taking advantage of the higher oxidation state of Os(IV) relative to Os(II); the inventors found that Osmium (IV) complexes having more reasonable HOMO levels. i.e. a HOMO level around −5.0 eV. For example, comparative compounds 1 and 2 in Table 2, which are the osmium (II) complexes; the HOMO level is −4.3 eV and −4.52 eV individually. On the other hand; the rest of compounds in Table 2, which are the osmium (IV) complexes, the HOMO level is around −5.0 eV. Moreover, depending on the ligand environment, osmium (IV) can adopt face-capped octahedral, pentagonal bipyramidal, square-face capped trigonal prism; or trigonal-face capped trigonal prism configuration which can result in better flexibility for choice of ligands, making them suitable for display applications.

It is understood that the various embodiments described herein are by way of example only, and are not intended to limit the scope of the invention. For example, many of the materials and structures described herein may be substituted with other materials and structures without deviating from the spirit of the invention. The present invention as claimed may therefore include variations from the particular examples and preferred embodiments described herein, as will be apparent to one of skill in the art. It is understood that various theories as to why the invention works are not intended to be limiting. 

We claim:
 1. A compound having the formula Os(L)_(n), wherein Os is osmium(IV) metal, L is a ligand coordinating to Os, and n is an integer from 1 to 6; wherein each L can be same or different; wherein at least one L is a multidentate ligand; and wherein the compound is neutral, wherein (i) the multidentate ligand is selected from the group consisting of:

or (ii) the compound has a formula

wherein X is a neutral coordinating atom selected from the group consisting of carbene, phosphorus, and nitrogen; wherein C is an anionic coordinating carbon atom; wherein N is an anionic coordinating nitrogen atom; wherein O is an anionic coordinating oxygen atom; and wherein H is hydride.
 2. The compound of claim 1, wherein the neutral carbene is N-heterocyclic carbene; wherein the neutral phosphorus is phosphorus atom of a trisubstituted phosphine; and wherein the neutral nitrogen is sp² nitrogen atom of N-heterocyclic ring selected from the group consisting of pyridine, imidazole, benzoimidazole, pyrazole, and triazole.
 3. The compound of claim 1, wherein the anionic coordinating carbon is sp² carbon atom selected from the group consisting of benzene, pyridine, furan, thiophene, and pyrrole.
 4. The compound of claim 1, wherein the anionic coordinating nitrogen is sp² nitrogen atom of N-heterocyclic ring selected from the group consisting of imidazole, benzoimidazole, pyrazole, and triazole.
 5. The compound of claim 1, wherein the anionic oxygen atom is oxygen atom from carboxylic acid or ether.
 6. The compound of claim 2, wherein one L is a tetradentate ligand.
 7. The compound of claim 6, wherein the compound has a formula selected from the group consisting of:


8. The compound of claim 7, wherein the compound is selected from the group consisting of


9. The compound of claim 1, wherein the compound has a formula

wherein H is hydride.
 10. The compound of claim 9, wherein the compound is selected from the group consisting of


11. A first device comprising a first organic light emitting device, the first organic light emitting device comprising: an anode; a cathode; and an organic layer, disposed between the anode and the cathode, comprising a compound having the formula Os(L)n, wherein Os is Os(IV) metal, L is a ligand coordinating to Os, and n is an integer from 1 to 6; wherein each L can be same or different; wherein at least one L is a multidentate ligand; and wherein the compound is neutral, wherein (i) the multidentate ligand is selected from the group consisting of:

or (ii) the compound has a formula

wherein X is a neutral coordinating atom selected from the group consisting of carbene, phosphorus, and nitrogen; wherein C is an anionic coordinating carbon atom; wherein N is an anionic coordinating nitrogen atom; wherein O is an anionic coordinating oxygen atom; and wherein H is hydride.
 12. The first device of claim 11, wherein the first device is selected from the group consisting of a consumer product, an organic light emitting device, and a light panel.
 13. The first device of claim 11, wherein the organic layer is an emissive layer and the compound is an emissive dopant.
 14. The first device of claim 11, wherein the organic layer is an emissive layer and the compound is a non-emissive dopant.
 15. The first device of claim 11, wherein the organic layer further comprises a host material.
 16. The first device of claim 15, wherein the host material comprises a triphenylene containing benzo-fused thiophene or benzo-fused furan; wherein any substituent in the host material is an unfused substituent independently selected from the group consisting of C_(n)H_(2n+1), OC_(n)H_(2n+1), OAr₁, N(C_(n)H_(2n+1))₂, N(Ar₁)(Ar₂), CH═CH—C_(n)H_(2n+1), C≡CC_(n)H_(2n+1), Ar₁, Ar₁—Ar₂, C_(n)H_(2n)—Ar₁, or no substitution; wherein n is from 1 to 10; and wherein Ar₁ and Ar₂ are independently selected from the group consisting of benzene, biphenyl, naphthalene, triphenylene, carbazole, and heteroaromatic analogs thereof.
 17. The first device of claim 15, wherein the host material comprises at least one chemical group selected from the group consisting of carbazole, dibenzothiophene, dibenzofuran, dibenzoselenophene, azacarbazole, aza-dibenzothiophene, aza-dibenzofuran, and aza-dibenzoselenophene.
 18. The first device of claim 15, wherein the host material is selected from the group consisting of:

and combinations thereof.
 19. The first device of claim 15, wherein the host material comprises a metal complex.
 20. A formulation comprising a compound having the formula Os(L)n, wherein Os is Os(IV) metal, L is a ligand coordinating to Os, and n is an integer from 1 to 6 representing a maximum number of the L ligands that can coordinate to Os; wherein each L can be same or different; wherein at least one L is a multidentate ligand; and wherein the compound is neutral, wherein (i) the multidentate ligand is selected from the group consisting of:

or (ii) the compound has a formula

wherein X is a neutral coordinating atom selected from the group consisting of carbene, phosphorus, and nitrogen; wherein C is an anionic coordinating carbon atom; wherein N is an anionic coordinating nitrogen atom; wherein O is an anionic coordinating oxygen atom; and wherein H is hydride. 